Hitherto, a chlorine-containing hydrocarbon, such as dichloromethane, has been used as an organic solvent of a cellulose acylate solution used when forming a cellulose acylate film used in a silver halide photographic photosensitive material or a liquid crystal image display device. Dichloromethane (boiling point, about 40° C.) has been conventionally used as a good solvent for cellulose acylate. Dichloromethane is a preferable solvent due to its advantage of easy drying in film-forming and drying steps in the process for producing an article as described above, because it has a low boiling point. In recent years, as to chlorine-containing organic solvents having a low boiling point, leakage thereof has been remarkably reduced in the step of handling the solvents, even in airtight facilities, from the viewpoint of protecting the environment. For example, an exhaustive closed system to prevent leakage from the system has been developed. Even if the organic solvent is leaked, the following method is adopted to prevent the hydrocarbon from being discharged outdoors: installing a gas-absorbing tower to absorb the organic solvent, and treating the solvent before discharge. This method also has been improved. Furthermore, before discharge, the chlorine-containing organic solvent is decomposed by burning based on thermal power, or by action of electron beams. In this way, the organic solvent has hardly been discharged outdoors. However, it is necessary to conduct further research to attain complete prevention of discharge.
Attempts have been made to find solvents for cellulose acylate that are different from dichloromethane, which has been favorably used as a chlorine-containing organic solvent. Examples of known organic solvents in which cellulose acylate, in particular cellulose triester, can be dissolved include acetone (boiling point, 56° C.), methyl acetate (boiling point, 56° C.), tetrahydrofuran (boiling point, 65° C.), 1,3-dioxolane (boiling point, 75° C.), and 1,4-dioxane (boiling point, 101° C.). Of these examples, methyl acetate is excellent in solubility and film-forming property.
Generally, a film is produced by a method in which a dope (a concentrated polymer solution) using the aforementioned organic solvents is cast on a band and peeled from the band, followed by drying with conveying. Methods for producing a film from a dope prepared with methyl acetate are described in JP-A-2002-192541, JP-A-2002-160242, and JP-A-2003-55476. These methods are primarily directed at improving the solubility of a polymer, which results in improvements in each of long-term continuous productivity of the film, storage stability of the dope, and ease of peeling from the band.
However, in each of these methods, optical unevenness (i.e., unevenness of retardation (Rth) in the thick direction; the term “retardation in the thick direction” means the value of the difference in an average refractive indices in the casting direction (MD) and the width direction (TD) and the refractive index in thick direction (TH); multiplied by the thickness of the film) is large. Further, uneven thermal expansion are apt to arise.
In addition, in each of these methods, a formed film of the dope is low in elastic modulus at the time of peeling, and stretch is easily caused by tension during conveyance of the formed film after peeling. From these defects, optical unevenness (i.e., unevenness of retardation (Re) in the face; the term “retardation in the face” means the value of the difference in refractive indices of the film between the casting direction (MD) and the width direction (TD), multiplied by the thickness of the film) and uneven thermal shrinkage are apt to arise.
The production of cellulose acylate films by a cast method is classified into a method called “drum casting” which comprises steps of casting the prepared cellulose acylate solution onto a chilled drum, cooling it to a temperature ranging from 15° C. to −100° C., for gelling, and then peeling the gelled cellulose acylate, followed by drying the peeled film; and a method called “band casting” which comprises steps of casting the prepared cellulose acylate solution onto a band heated at a temperature ranging from 15° C. to 50° C., for drying, and then peeling the dried film. However, the aforementioned problems have been remarkable in drum casting.
When Rth unevenness is large, in case that a cellulose acylate film is used for a large-size liquid crystal display plate, irregularity (unevenness) of display resulting from the uneven Rth is easy to occur. In particular, when the large-size liquid crystal display plate is seen from the oblique direction thereof, this problem is remarkable. In addition, if the uneven thermal expansion is severe, uneven stress to the expansion occurs on account that the heat expansion owing to thermal generated from light sources is locally different, which easily leads to occurrence of optical unevenness owing to photo-elasticity. Therefore, improvement on these defects has been desired.
When a cellulose acylate film is used for a large-size liquid crystal display plate, irregularity (unevenness) of display resulting from uneven Re easily occurs. In addition, if uneven thermal shrinkage is severe, uneven stress to the shrinkage occurs, because the thermal shrinkage owing to heat generated from light sources is locally different, which easily leads to occurrence of optical unevenness owing to photoelasticity.
Further, when a cellulose acylate film is used for a silver halide photosensitive material, such uneven thermal expansion or uneven thermal shrinkage easily causes image distortion during long-term preservation. In other words, a major cause for these problems is a shortage of strength when a dope is cast. Therefore, improvement on these defects has been desired.